I. Field of the Invention
The present invention relates generally to sheet metal hemming machines.
II. Description of Related Art
There are many previously known hemming machines. Many industries, such as the automotive industry, utilize sheet metal hemming machines to secure two metal parts together. These sheet metal hemming machines typically comprise a base having a nest vertically slidably mounted relative to the base. The nest, in turn, supports the part to be hemmed.
At least one, and typically three to five hemming die sets are laterally slidably mounted to the base and movable between an extended position and a retracted position. In the extended position, the die overlaps the nest so that vertical displacement of the nest toward the hemming die causes the part to be hemmed to be compressed upon the die thus forming the hem. Typically, a prehem is first formed by a prehem die to bend the sheet metal at an angle of approximately 45xc2x0 while a final hem die retrorsely flattens the sheet metal hem together.
In order to form the hem, the part to be hemmed is first positioned on the nest and, with the hemming dies retracted, the nest is moved to a position just below the prehem die and clearing the part flange to be hemmed. The prehem die set is then moved to an extended position after which the nest is displaced vertically upwardly against the prehem die and retracted after having reached the nominal hemming pressure. The hemming dies are then moved to a retracted position and the nest is moved to a position just below the final hem die. The final hem die is then moved to an extended position and the nest is vertically displaced against the final hem die to complete the hem and also retracted after having reached the final hem pressure. The dies are then moved to their retracted position and the finished part is removed from the nest.
These previously known hemming machines have all suffered from a number of disadvantages. One disadvantage is that the previously known hemming machines have required the use of multiple hydraulic actuators to vertically displace the nest due to the massive weight of the nest. Such actuating means are expensive, hard to maintain and polluting.
Derivated from the previously already known machines, a first generation of electric hemmer has been developed by simply replacing the hydraulic cylinders by one or more linear ball screws powered by electronically synchronized drives.
But to face the double constraint of high production rate and high hemming pressure force, these drive configurations are generally oversized to be able to move quickly for a prehem to a final hem position in high speed, and then to deliver a high torque in static. Such oversizing (x4; x6) is not only expensive, but presents a real risk for the tooling in case of jamming or other incidental event, by introducing a tremendous reverse inertia to the system.
The present invention provides a hemming machine which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, the hemming machine of the present invention comprises a base which is fixed to a ground support surface. Both a nest and nest carrier are vertically slidably mounted to the base with the nest carrier positioned beneath the nest. In the conventional fashion, the nest is adapted to support the part to be hemmed.
Similarly, in the conventional fashion, at least one, and more typically three to five sets of dies, are laterally slidably mounted to the base between an extended and a retracted position. In their extended position, the dies overlie the nest and thus the part to be hemmed. Conversely, in their retracted position, the dies are laterally spaced from the nest to permit free vertical movement of the nest past the dies as well as the part loading/unloading. One die set typically performs the prehem while the other die forms the final hem.
The nest carrier and nest are vertically movably mounted not only relative to the base, but also relative to each other. In order to displace the nest relative to the nest carrier, at least one hydraulic driven bladder is sandwiched in between the nest and the nest carrier while a lock unit selectively locks the nest carrier against downward movement. Thus, with the nest carrier locked against vertical movement, inflation of the driven bladder vertically displaces the nest upwardly relative to the nest carrier.
In order to selectively inflate and deflate the driven bladder, at least one drive bladder is sandwiched in between a piston and the nest. This drive bladder is fluidly connected to the driven bladders by fluid conduits. Thus, with the nest carrier locked against downward vertical movement, movement of the piston toward the nest compresses the drive bladders thus pumping hydraulic fluid contained within the drive bladders from the drive bladders and to the driven bladders. This in turn vertically displaces the nest upwardly relative to the nest carrier so that, with the hemming dies in their extended position, the part to be hemmed is compressed against the hemming dies in the desired fashion.
In the preferred embodiment of the invention, a plurality of pressurized pneumatic spring (or air) bladders are sandwiched between the base and nest carrier and urge the nest and nest carrier upwardly. Additionally, a single rotary shaft is rotatably mounted to the base and extends through an opening in the nest carrier and threadably engages the piston. The piston in turn abuts against an upper surface of the nest carrier thus holding the nest carrier against upward movement due to the force developed by the pneumatic spring bladders.
With the lock unit in its retracted position so that both the nest carrier and nest can move vertically relative to the base, rotation of the shaft vertically moves the piston upwardly so that the inflation of the air spring bladders likewise moves both the nest and nest carrier upwardly in unison with each other in order to position the upper surface of the nest beneath either the prehem or final die. When the nest is so positioned, one or more lock units engage the nest carrier to preclude downward movement of the nest carrier.
Thereafter, continued rotation of the shaft in the same direction continues to move the nest together with the part to be hemmed upwardly until the part engages the hemming die. When this occurs, the upward movement of the piston encounters an increasing downward force due to the compression of the part against the hemming die. When this occurs, the continued upward movement of the piston causes the piston to separate from the nest carrier and, in doing so, compress the drive bladders. This in turn inflates the driven bladders with amplified force thus displacing the nest upwardly from the nest carrier and performing either the hem or prehem operation.
The surface ratio between the drive and driven bladders is such that the force developed by the drive bladder is amplified by a factor of four to eight.